kbuild: fix kernel-devel RPM package and linux-headers Deb package

Since commit fe66b5d2 ("kbuild: refactor kernel-devel RPM package
and linux-headers Deb package"), the kernel-devel RPM package and
linux-headers Deb package are broken.

I double-quoted the $(find ... -type d), which resulted in newlines
being included in the argument to the outer find comment.

  find: 'arch/arm64/include\narch/arm64/kvm/hyp/include': No such file or directory

The outer find command is unneeded.

Fixes: fe66b5d2

 ("kbuild: refactor kernel-devel RPM package and linux-headers Deb package")
Reported-by: Karolis M <k4rolis@protonmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>